Alpha-methylstyrene-type monomers have been polymerized using solution polymerization procedures employing monofunctional anionic initiators such as the organolithium initiators. Polar additives have been employed to improve the rate of polymerization of such as alpha-methylstyrene. Subsequently, butadiene has been added to form a block of such as polybutadiene. The resulting product can be coupled, if desired. This generalized procedure is described in such as U.S. Pat. No. 3,825,623. Such polymers are useful in molded goods, hot melt adhesives, and other purposes.
Unfortunately, the prior art processes have been faced with a major problem of low conversion of the alpha-methylstyrene-type monomer, such as described in U.S. Pat. No. 3,825,623 at column 3.
An alpha-methylstyrene-type monomer, when polymerized anionically, establishes a reaction equilibrium between monomer polymer. Additional monomer does tend to force the formation of additional polymer; or, the use of low temperature does permit the formation of additional polymer with lower monomer presence, in effect tending to push the reaction toward the desired polymer. However, in order to effectively polymerize a conjugated diene thereon, the polymerization temperature must be raised. The natural tendency is for the alpha-methylstyrene-type monomer:polymer equilibrium to immediately return in the direction of unreacted monomer, thus depolymerizing the poly(alpha-methylstyrene-type monomer) block. This can occur with unseemly rapidity. This difficulty means variable sizes of poly(alpha-methylstyrene-type monomer) block, inadequate incorporation of the alpha-methylstyrene-type monomer, more monomer remains present after the conjugated diene polymerization, and monomers must be separated and recycled for economy. Sometimes the separation procedures from the polymerization diluent are difficult.
What is needed is a method to enforce good substantial conversion in the polymerization of the alpha-methylstyrene-type monomer, and then to hold it, avoiding depolymerization, while neatly copolymerizing thereon the conjugated diene and at the same time further copolymerizing any remaining alpha-methylstyrene-type monomer to relatively high conversion.